Fernando Verdasco is out of the German Tennis Championships after losing to world No. 156 Thiago Monteiro in Hamburg.
Eighth seed Verdasco converted an early break point to take the first set but fatigue crept into his game as he struggled to match his opponent in longer rallies.
At 6-5 down and serving to force a tiebreak, the Spaniard committed four unforced errors to concede defeat after two hours and 10 minutes of play.
World No. 146 Monteiro will look to reach only his second semifinal of the season when he takes on Slovakia's Jozef Kovalik, who raced past German Rudolf Molleker 6-4 6-0.
Elsewhere, second seed Deigo Schwartzman continued his strong form on clay and also advanced after a 6-2 6-2 victory over Germany's Daniel Masur.
Schwartzman treated the crowd to a display of power hitting as the Argentine capitalised on two breaks of serve in each set to wrap up the victory.
In the final match of the day, Leonardo Mayer beat Gael Monfils 6-1 7-5 to set up a quarterfinal clash with Schwartzman, who is currently the world No. 12.
Meanwhile in Gstaad at the Swiss Open, fourth seed Andrey Rublev was defeated by Matteo Berrettini 6-3 6-3.
The 20-year-old, who is currently ranked 46th in the world, had received a bye into the second round but was beaten by Berrettini in one hour and four minutes.
Second seed Roberto Bautista Agut came from a set down to defeat fellow Spaniard Jaume Munar 2-6 6-3 6-2.
Japan's Taro Daniel also advanced after beating Roberto Carballes Baena 6-4 4-6 7-6 (4).
In the final match of the day Feliciano Lopez beat Oriol Roca Batalla 4-6 7-6 (4) 6-4.